The etiological features are no serious trauma history and long-term overwhelming training history. In the clinical manifestation, the histogram display mainly shows feeling pain during exercise, the Lachman test is positive and MRI image changes. In addition, this MRI changing needs to pay more attention. ACL fatigue lesion is mostly combined with a small amount of effusion in joints and synovitis in MRI. The ACL signal is mainly manifested as thinning, thickening, tension change, etc. And can also incorporate the adaptive change of PCL. We believe that thinner ACLs would be injured easier at the same training intensity, which will lead to blurred ACL edges and tension changes. The partial ACL damage has led to changes of biomechanical structure in the joint, which has caused synovitis and joint effusion. Further research on ACL fatigue lesion image changes will be carried out by our team soon.
Some research has pointed that fatigue could cause acute ACL lesion [12-23], but the definition of ACL fatigue lesion has not been revealed completely from academic field. Until 2013, Ashton-Miller 's biomechanical study based on knee model through donated human corpses has demonstrated that repetitive pressure-applying jump is likely to cause ACL fatigue damage, but Miller 's research is only limited in biomechanical field, not involved with clinical research [24-25]. For two of the cases, arthroscopy surgery was performed and concluded that ACL fatigue lesion without trauma history manifested as ligament relaxation and partial femoral end damage.
Regarding the causes of chronic ACL fatigue lesion, the exploring is still on process. However, with the academic theory, there are many ACL lesion-causing factors that can explain its development from different aspects. For example, the signal receptor on ligament surface hypothesis, abundant mechanical receptors exist on ACL surface, especially the ligament end points and synovial folds [26]. Dhillon MS's analyzed the risk of the cruciate ligament damage within the changes of mechanical receptors. His research confirmed that the number of mechanical receptors was related to the prolonged ligament lesion, the longer the damage, the less the number of mechanical receptors [27]. After reviewing the literature and considering clinical observations, we state a hypothesis that fatigue can weaken the muscle force surrounding the knee, thereby weakening the dynamic stability of the knee. At the same time, knee static stability structure most relied on ligament stands outranged tension, secondly makes ligaments deformed, mechanical receptors decreased, eventually the chronic ACL lesion happens. More importantly, this process can also lead to another kind of positive feedback cycle, that is, chronic lesion leads to static damage, and static damage exacerbates chronic lesion. This also theoretically explains the root cause of that one case who was diagnosed with chronic ACL fatigue lesion, he was admitted to our hospital again for acute complete ACL rupture due to sprain during training after discharging in six months. Other research groups had also clarified their views based on other case-control studies and biomechanical analysis, under fatigue, the flexion and valgus angles of the hip, knee and ankle joints are different from the normal state, so that it changed the force from ground to the body[28], converting the knee torque in various directions, further proving fatigue condition induces ACL lesion.
Swanik et al [29]. also reported that athletes maintained a slower response and a slower processing speed in the non-contact ACL lesion compared with the control group. Many cases we saw in the clinic, including the individual case with chronic ACL fatigue lesion reported this time, all of them have experienced long-term high-intensity overwhelmed training, besides, warm-up preparation before training, fatigue recovery status between training intervals and after training Stretching movements are all lack of scientific guidance. More reports confirm that during fatigue status, the neuromuscular control and coordination ability become inactive, neuromuscular control is weakened, ligament tension and joint pressure are altered, increasing the risk of ACL rupture [30].
Furthermore, another theory is the cruciate ligament will gradually degenerate with the development of knee osteoarthritis by age growing, accompanied with decreasing ACL function [31] and knee proprioception [32]. The cruciate ligament degeneration is mainly associated with inflammation, which is an active, cell-mediated process of extracellular matrix reduction and remodeling [33]. The degeneration of ligaments and the weakening of proprioception of knee joint will promote the further development of osteoarthritis, which complement each other [34]. This theory could also explain our finding that chronic ACL fatigue lesion is more common in sporty people over the age of 30, while are rare in patients under the age of 25.
Certainly, chronic ACL injuries also share common risk factors with acute ACL injuries, such as ACL development defects, intercondylar fossa abnormalities, BMI, gender, and multiple joint relaxations [35-41]. Our study also supports this theory. Through univariate analysis, we know that ACL fatigue failure is related to the patient's age, daily training intensity, BMI, military service time, and knee hyperextension, such as knee function decreasing along with aging. Through multivariate analysis, it is known that weekly training intensity, BMI, and knee hyperextension all are independent influence factors for ACL fatigue lesion.
There are also some limitations in this report, due to the difference of individual physical quality and the complexity of qualifying fatigue, the evaluation of patients’ fatigue degree can’t be completely accurate. The content and intensity of the training vary with the occupation of the patients. This paper is only a preliminary clinical observation study, which can guide us to further understand the manifestations of such diseases and provide help for more targeted research in the future.
In conclusion, ACL fatigue lesion is mainly manifested by pain and instability of the knee joints, especially the exercising pain. In patients’ physical examinations, the positive proportion of Lachman test was as high as 84.9%, and very few (5) were positive in axial movement test, which proved that most patients presented with partial and mild lesions rather than severe ACL relaxation or even rupture. The high incidence of synovitis and intra-articular effusion was observed on MRI, both of which were the result of long-term high-intensity training.